Private documents, spell books, anything you don't want to keep others away from are kept in this chest.
when opened without going thought the proper sequence (twist key left then right, tap side twice, ect.) when opened the contents of the chest are engulfed in flames or acid, possibly harming the person opening the box.
However some items are too valuable to destroy out right, so some of these boxes have a false bottom that is protected from the effects of opening the box.
Burn boxes with false bottoms works mostly though the misleading as most thieves, spy and annoying adventures as they will believe that all objects of value are gone. Only the most observant will find the false bottom, most will only find the remains of other less valuable doucuments.